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1 SIM card = 1 line. You can switch that SIM card between devices all day long, but the device with the SIM card in it will make & receive calls & texts.

You can check your voicemail by calling 805-637-7243 from any phone, then entering your phone # and password. You will only be notified of new voice messages on the phone which contains the SIM card, however.

You can also get a Google Voice account & phone #. You can use your Google Voice account to make & receive calls on multiple devices.

I believe you can port your existing phone # to Google Voice for a fee, but that will automatically cancel your T-Mobile line. If you want to port your T-Mobile # to Google Voice and get a new # on T-Mobile, add a second line to your account (assuming you're on an individual plan) before porting your old # to Google Voice.

I don't know how the technology works. Samsung may create something similar in the future, but it would probably mean Samsung devices would only talk to each other, so if you had a Sammy watch and an HTC phone it may not work?

I think we are not referring to the same feature. I was thinking this multiple devices will works like ATT's number sync, basically, your phone and wearable device in this case Gear S2, will have basically the same number and will ring at the same time. In today's T-mobile Gear S2, the watch has it's own number and when you set it up to call forward your phone to the watch via Samsung Gear App, the phone rings for about a few seconds and forwards the call to the watch.

If I remember it correctly, a t-mobile executive mentioned that they are working on a feature similar to ATT's number sync, but better. That was probably 6 months ago.

So is there any update on T-Mobile's numbersync? I'd really like to add the Samsung Gear S2 or upcoming S3 to my T-Mobile account, but I don't see the point if I cannot share my main line's number for calling and messaging.